Sickle cell disease and other rigid hemoglobin disorders are due to the tendency of slightly abnormal molecules to develop intimate molecular associations. The closer degrees of molecular interaction can cause increased viscosity and rigidity of normal red blood cells. If the molecular association leads to the development of polymerization it can cause severe distortion of erythrocyte shape and lead to alterations in the red cell membrane favoring aggregation of the cells into masses that occlude the microcirculation. Investigations in this laboratory are aimed at defining the mechanisms of hemoglobin molecular interaction by evaluation of various phases of hemoglobin association, aggregation, molecular assembly into polymers, and conversion into crystals in intact erythrocytes, in cell-free solutions from patients with sickle cell disease, in cells and hemolysates from patients with non S hemoglobinopathies, and in a variety of acquired and inherited rigid hemoglobin syndromes in man and animals. The methods employed for this investigation include morphological study of red cells and hemoglobin polymers with the aid of scanning electron microscopy and transmission electron microscopy. For transmission electron microscopic studies the procedures of negative stain whole mount, thin sectioning and freeze etching are applied in a broad range of approaches to understanding the basic molecular composition of hemoglobin polymers and crystals. Biochemical studies include methods for evaluating the purity of hemoglobin solutions and studying the enzyme composition and adenine nucleotide balance in normal and affected erythrocytes. The morphological and biochemical studies are combined in many experiments in order to develop a total understanding of the pathology of rigid hemoglobin syndromes.